1. Technical Field of the Invention
The present invention relates to the inspection of surface structures on an object and, more specifically, to a system and method for optically inspecting the surface of an object having surface structures thereon.
2. Description of Related Art
Optical inspection of an article of manufacture encompasses a variety of techniques that make use of the patterns produced by energy reflecting off (or passing through) the object being inspected. These reflections constitute an image that can be examined, compared with other images, stored, and otherwise analyzed. Examination may be performed by the human eye, with or without the aid of a magnifying device, such as a microscope. Or the image may be `captured` by a camera and converted directly into electronic data for storage and analysis, either immediately or at some time in the future. In the manufacturing process, optical inspection can be done for the purpose of final acceptance or rejection, or as an intermediate step so that correctable defects can be remedied.
An example of a useful application of the present invention is the inspection of semiconductor wafers ("wafers"), both during and after the manufacturing process. Semiconductor wafers are slices of a semiconducting material, such as silicon, that are repeatedly coated, treated, and etched away in selected areas to form small (usually very small) interconnected electronic devices, such as transistors. A set of thousands, even millions of these interconnected devices is then separated from other sets formed on the wafer and encased in a package to form what is commonly referred to as a "chip". Chips can contain a very large number of electrical circuits and are used in a wide variety of applications. A single such chip, for example, one called a microprocessor, forms the "brains" of a modern personal computer.
A single wafer can serve as the base for forming several, or even hundreds of such devices. In order to transform a wafer into a set of microprocessors or other electronic devices, the wafer undergoes several manufacturing steps. First, a wafer is cut from a crystal ingot (such as crystallized silicon), and an epitaxial layer (a single layer of silicon crystals) may typically be grown on it. The creation of an epitaxial layer is often followed by the growth of high quality oxides on the wafer surface in a process called oxidation. Next, the wafer undergoes several fabrication steps. Each fabrication step places a layer of ions or other materials into or on the wafer, or removes portions from it, in a predetermined geometric pattern so as to form a portion of an electronic circuit. When these fabrication steps are completed, the wafer surface typically possesses several functional microelectronic devices. Each area of the wafer that is to become a separate device is called a "die".
Common wafer fabrication steps include chemical vapor depositions (CVD), plasma-enhanced vapor depositions (PECVD), etches, ion implantations, diffusions, metalizations, or the growth of structures directly on the wafer. Naturally, these structures are quite small, and the successful completion of the fabrication steps depends largely on the ability to precisely control the geometric placement of gasses, ions, metals, or other deposition materials. The processes of etching, implanting, etc., must be done with sub-micron precision. The precise placement of ions, metals, gasses, or other deposits and removal of other materials is often achieved through a process called photolithography. Though photolithography is well known in the art of microelectronic device manufacturing, a brief description thereof is provided herein because the teachings of the present invention are more particularly exemplified in the context of semiconductor fabrication.
Photolithography is a process by which the wafer surface is selectively covered with a material called photoresist (or simply resist) so that subsequent processes of ion implantation, etching, etc., effect only certain areas. Using a technique similar to film development in photography, a geometric pattern is transferred from a negative known as a mask (also called a reticle) onto a wafer. Photoresist contains photoactive sensitizers, and exposing it to light (or other activating radiation) produces a chemical change. The pattern in the mask causes light passing through it to be selectively blocked out, in effect casting a precise shadow onto the resist. The desired chemical change then occurs in the exposed areas. This step of the process is called imaging.
The first step in photolithography, however, is the preparation of the wafer itself. The wafer is cleaned, and a thin layer of liquid photoresist is distributed evenly across the top surface of the wafer. The photoresist is dried, and then the wafer with the photoresist thereon is heated to vaporize any solvents. Imaging can now be performed. During the imaging step, the photoresist is exposed to a light source at a predetermined wavelength. The mask's pattern, like a photographic negative, is projected onto one portion of the wafer at a time by a precision optical device known as a "stepper", and the pattern is preserved on each die by the photoresist. There are different kinds of photoresist used in wafer manufacture, each having different properties. "Positive" photoresist, for example, is made soluble by exposure to the light, while "negative" photoresist is hardened.
The next step in photolithography is called development, where the wafer is flushed with a solvent that washes away certain portions of the photoresist. The types of solvents used also varies. One solvent will wash away the portions of positive photoresist that were exposed to the light, while another washes away the unexposed portions of negative photoresist. In either case, the development process leaves the geometric pattern of the mask (or its negative) on each die. The result is a series of "photoresist structures" that together constitute a developed photoresist layer.
By selectively covering portions of the semiconductor wafer with photoresist structures, the entire wafer can, in a subsequent fabrication step, be exposed to various chemicals, ions, metals, or etchings without affecting the entire areas under the photoresist structures. After each fabrication step has been completed, a wash step is executed. In the wash step, all remaining photoresist is washed away and the wafer is cleaned. Often, one or more additional fabrication steps will be needed, and, the wafer will then undergo further photolithography processes.
When all fabrication steps are complete, the electrical characteristics of each die are tested. Based on the results of these tests, the die are "binned" (i.e., they are classified as good or defective). The wafer is then sliced into separate dice which are sorted to discard defective ones. The good dice are then prepared for packaging.
As can be seen from this discussion, in order to correctly manufacture microelectronic devices, geometrically correct patterns of photoresist structures must be deposited on the wafer during fabrication. And correct geometric patterning is dependent upon properly imaging and developing photoresist layers.
Each fabrication step is expensive and adds significantly to the cost of the semiconductor wafer. Furthermore, fabrication steps such as etching and ion implantation are difficult, if not impossible, to reverse in any cost-effective way. By contrast, photoresist structures can be removed quickly and with minimal disturbance to the underlying wafer structures. Thus, it is desirable to detect defects in the developed photoresist prior to performing a fabrication step. Photoresist defects are those anomalies that will result in impaired or altered electrical characteristics when fabrication of the die is complete, causing it to be rejected. Common photoresist defects include alignment errors, missing photoresist structures, contamination, and skewed photoresist.
If a defect can be detected in the developed photoresist layer prior to a fabrication step, one simply washes away the photoresist structures and develops another photoresist layer in place of the defective one. If the number of defects attributable to imperfections on the photoresist can be thereby reduced, the corresponding increase in die yield will result in considerable savings.
The most common method used to detect imperfections in a developed photoresist layer is optical inspection. Other available methods include electronic, ion beam, and X-ray imaging, but they are slower and more expensive than optical inspection because these imaging techniques illuminate and reconstruct only one point at a time. Laser imaging techniques that capture and compare the angle of reflection of laser beams can also be employed, but they lack comparable precision in reporting the position of defects.
Optical wafer inspection devices typically employ a support that holds a wafer under an overhead camera and one or more sources of light. In operation, the optical inspection device generally lights the wafer from several directions in order to provide full illumination, and the overhead camera captures a gray-scale (black-and-white) image of the wafer with a developed photoresist layer thereon. The captured image can then be analyzed in a variety of ways, generally with the aid of (or entirely by) a computer.
Although numerous advances have been recently achieved in the optical detection, analysis, and classification of defects, present systems still have disadvantages associated with efficiency, accuracy, and adaptability in manufacturing. Greater success in these areas can be obtained through the use of optimally configured illumination of the object being inspected. The system and method of the present invention have been discovered to lead to just such a result.